The present invention relates to a preparation of sorbic acid, and more particularly to a method for preparing sorbic acid of high quality in high yield.
Sorbic acid has been manufactured by reacting crotonaldehyde with ketene to form polyester and converting the polyester to sorbic acid by means of heating, or in the presence of an alkali or mineral acid. A method employing a mineral acid, especially hydrochloric acid, has been advantageously applied because the yield or the quality of obtaines sorbic acid is superior. There has been known, for instance, a method of splitting the polyester in diluted or concentrated hydrochloric acid and then recovering the sorbic acid deposited in the reaction mixture, or a method of splitting the polyester in the presence of hydrochloric acid and a water-miscible organic solvent such as acetic acid or dioxane and then recovering sorbic acid from the solvent.
However, in the case of splitting the polyester by a diluted hydrochloric acid, the yieldd of the obtained sorbic acid is at most about 85 %. In the case of employing a concentrated hydrochloric acid (around 35 % by weight), the yield reaches about 90 %. But, in such a method by-production of a colored resinous substance is inevitable during the reaction and, therefore, the method requires the particular purification step to remove the colored resinous substance. Such a contamination deteriorates the quality of sorbic acid. In order to purify the sorbic acid obtained in such a method, there has been applied recrystallization in the presence of active carbon. This complicates the process steps and the loss of sorbic acid due to adsorption on active carbon is considerable.
In the case of splitting the polyester in a water-miscible organic solvent, the reaction proceeds homogeneously and the obtained sorbic acid is relatively good in quality. However, the method requires a procedure such as distillation to remove a large quantity of the solvent or addition of a non-solvent, e.g. water, to the reaction mixture, because the formed sorbic acid is dissolved in the solvent in large quantities. Moreover, the method has a considerable difficulty in recovering the hydrochloric acid employed.
For the purpose of overcoming the defects in a conventional method as mentioned above, the present inventors have proposed a method for preparing sorbic acid which consists of the steps of uniformly dispersing the polyester and a small quantity of hydrochloric acid into a saturated aliphatic hydrocarbon having a boiling point at atmospheric pressure of 90.degree. to 180.degree.C., heating the dispersed reaction mixture at a temperature of 80.degree. to 135.degree.C., and recovering the sorbic acid precipitated by cooling the reaction mixture. This method has the advantages that the amount of hydrochloric acid is less than that in a conventional method, and that sorbic acid is produced in high yield. However, a colored resinous substance is dissolved in the hydrocarbon solution of sorbic acid and the sorbic acid precipitated from such a solution is also colored. Therefore, this method is not still satisfactory.